This application relates generally to bone growth stimulators, and more particularly to electrodes for use with implantable stimulators employing direct current for stimulation of bone growth.
Bone growth stimulators employing direct current are known to be useful as adjuncts to surgical procedures for treatment of poorly healing bone fractures known as nonunions, and as adjuncts to spinal fusion procedures among other applications. Direct current bone stimulation is performed by supplying a constant current to an electronegative cathode implanted at a bony site where fusion is desired. Stimulation of bone growth depends in part on maximal contact between the cathode surface and viable bone at the fusion site, which is normally prepared by debriding or decorticating host bone to expose live bone cells. It is preferable that the cathode contact both the host bone and any graft material placed at the fusion site.
Cathode flexibility, and in particular an ability to conform to the shape of a fusion site, is an important factor affecting the extent of such contact. Other factors include the roughness of the prepared surface of the fusion site, unevenness of the interface between the cathode and graft material packed against it, gaps between bone fragments at a nonunion site, and, in spinal fusion cases, open spaces between vertebrae to be fused together.
The performance of a bone growth stimulator can also be affected by pre-implant manipulations of the cathode by a surgeon or surgical assistant to make it conform to a fusion site. Cathode manipulations vary among surgeons and their assistants and can lead to inconsistent cathode configurations, and thus inconsistent results, from case to case. A cathode requiring little or no manipulation at the time of implantation is desirable, and it would be particularly desirable to have a cathode requiring minimal manipulation for a range of sizes of fusion sites.
Various electrode configurations are known for use in DC bone growth stimulation, including mesh electrodes as well as electrodes having straight, sinusoidal, or helical shapes. A mesh anode, for example, is disclosed in U.S. Pat. No. 3,842,841 to Brighton et al. The anode has a metal face plate 0.7 mm thick to which a stainless steel mesh is soldered. As such, the anode has little if any conformability and cannot be extended in length. U.S. Pat. No. 5,304,210 to Crook discloses a mesh electrode frame placed over a bone injury site for purposes of bone growth stimulation. The mesh is a single rectangular piece of generally flexible material with two rows of apertures formed therein. There is no indication that it is designed to stretch and it appears incapable of stretching.
Mesh cathodes are disclosed along with individual wire cathodes in U.S. Pat. No. 4,506,673 to Bonnell, for use in stimulating tissue growth in cartilage or ligaments. Bonnell refers to cutting or otherwise shaping a cathode to approximate the shape of a defect to be treated, and discusses rolling or folding of a mesh cathode for insertion followed by unrolling of the cathode and arthroscopically guided positioning thereof over the defect to be treated. Stretching of a cathode is not discussed, and the disclosed mesh cathodes, each in the form of a square-cell grid, are incapable of substantial stretching.
Electro-Biology, Inc. (EBI), the assignee of the present invention, manufactures a cathode of trifilar wire, i.e., three-filament stranded wire, designed to be manually shaped by a surgeon at the time of implantation to conform to the surgical site. The trifilar wire is typically manipulated into a zigzag or sinusoidal wave shape or into a helix. A helix may be formed with the aid of a stepped, cylindrical mandrel provided for such purposes, or by directly wrapping the wire cathode around a length of cortical bone graft. The helix configuration is typically placed in a trough or a drill hole extending across a nonunion, but may alternatively be inserted directly into a nonunion site between the bone surfaces to be stimulated. In a spinal application, the helix configuration is known to have been flattened and stretched to fit a desired fusion site.
A trifilar wire cathode with a sinusoidal wave shape for spinal fusion is also available from EBI as a prefabricated cathode, that is, a cathode formed into the desired shape at the time of fabrication. The preformed shape of this prior art cathode is shown in FIG. 2. In the example illustrated, the cathode has 12 cm of trifilar wire formed as shown to create a wave with an area of coverage, or footprint, having a nominal length (L) of 4 cm and a nominal width (W) of 1 cm. This cathode is also available in a 24-cm size having twice as many cycles of the wave as in the 12-cm size, and a footprint twice as long, i.e., 8 cm long. A surgeon adjusts the prefabricated, or factory-preformed, cathode in length to fit the length of a fusion site. The preformed wave shape provides consistency and reduces surgical preparation time, and its efficacy is well documented. Nevertheless, there remains a need for a cathode configuration that further improves the delivery of direct current to a bone fusion site while minimizing pre-implant manipulations.